Production process and adhesives for corrugated board

ABSTRACT

A process for production of corrugated board using an aqueous adhesive which contains polyvinyl alcohol and clay, or polyvinyl alcohol, clay and water-soluble boron compound, the ratio of clay to polyvinyl alcohol being in the range of 200 to 600 parts by weight based on 100 parts by weight of polyvinyl alcohol, the ratio of water-soluble boron compound being not more than 15 parts by weight based on 100 parts by weight of polyvinyl alcohol, the solid content of the adhesive being 25 to 60% by weight characterized in that 
     (a) a polyvinyl alcohol concentration in the solution excluding solid other than polyvinyl alcohol from adhesive being in the region bounded by the following formulas: 
     
         Y≧-0.0014 X+9.4                                     (i) 
    
     
         Y≦-0.0061 X+26.3                                    (II) 
    
     
         200≦X≦3500                                   (iii) 
    
      wherein Y is defined by the following formula: ##EQU1## P=Polyvinyl alcohol content in the adhesive (by weight) W=Water content in the adhesive (by weight) 
      and X is degree of polymerization of polyvinyl alcohol, 
     (b) the viscosity of the adhesive measured by a Brookfield viscometer being from 200 to 6000 centipoises at 85° C., and the rate of increase of the viscosity of the adhesive being in the range of 1.5 to 8 times when the temperature of adhesive is dropped from 85° to 30° C., 
     (c) the adhesive, of which temperature is 60° to 95° C., being applied onto the flute tips of corrugated medium to manufacture corrugated board, 
     and an aqueous adhesive therefor. The process for production of a corrugated board has excellent effect on heat energy saving characterized in that said corrugated board can be manufactured at high speed without heating after application of the water-base adhesive at a temperature of 60°-95° C., and the adhesive is easy to handle and has improved adhesion property.

FIELD OF THE INVENTION

The present invention relates to a heat energy saving and high speedproduction process for corrugated board, and to adhesives used for saidprocess. More particularly, the present invention relates to a processof manufacturing corrugated board having an excellent effect on heatenergy saving, characterized in that said corrugated board can bemanufactured at high production speed without heating after applicationof a water-based adhesive having a temperature of 60°-95° C., saidadhesive containing polyvinyl alcohol and clay, or polyvinyl alcohol,clay and a water-soluble boron compound, and being easy to handle andhaving improved adhesion.

BACKGROUND OF THE INVENTION

The manufacture of corrugated board essentially is to combine sheetlinerboards with a corrugated medium by means of adhesives. However, toomuch heat energy is consumed in this operation because water-base starchglue, for which gelatin and concentration are carried out only throughheating and drying, is used an adhesive.

In the manufacture of corrugated board, single-faced board is first madeon a single facer.

That is, fed sheet medium is corrugated while starch adhesive is appliedonto the tips of the flutes of said corrugated medium with an applicatorroll, and sheet linerboard is bonded, by means of heat and pressure, onsaid medium to which starch adhesive has been applied.

Then, the single-faced board manufactured on said single facer istransported to a glue machine, wherein starch adhesive is applied ontothe flute tips of said single-faced board, and is fed to between thecanvas and the heat tables heated to about 170° C. of a double facer.After said single-faced board is combined with fed double-facedlinerboard for single-wall corrugated board, or fed double-facedlinerboard and another single-faced board for double-wall corrugatedboard, weight rolls firmly press the combined board to canvas-coveredheat tables to gelatinize and dry the adhesive. Thereafter, theresulting single-wall board or double-wall board is slit and scored on aslitter scorer, is cut into a given length on a rotary cutoff, and isconverted into single-wall corrugated blanks or double-wall corrugatedblanks.

The adhesion on the double facer is carried out by heating starchadhesive applied onto the flute tips of the single-faced board throughdouble-faced linerboard for single-wall board or through double-facedlinerboard and another single-faced board for double-wall board.

Further, since single-faced board having formed flutes is combined withdouble-faced linerboard on the double facer, high pressure can not beloaded on the combined board because of possible flute damage. Thesecause heat transfer efficiency at the heating tables to be extremelylow. Therefore, much heat energy consumption in the double facer resultsin a severe problem.

In order to overcome the above defect, production processes forcorrugated board which employ adhesives requiring no heating have beenstudied these past years (See Japanese Patent Laid Open Publication Nos.32570/1981, 70074/1981 and 105952/1981). Furthermore, various studieshave been carried out for polyvinyl alcohol type adhesives for paper(See Japanese Patent Publication No. 22579/1961, Japanese Patent LaidOpen Publication No. 3336/1972 and U.S. Pat. No. 3,135,648 etc.).

However, as disclosed in the above-mentioned Japanese Patent Laid OpenPublication Nos. 32570/1981 and 70074/1981, the so-called adhesiveswhich can be applied to paper at a high temperature but which gelatinizeupon cooling are those which use a property that the adhesive applied ata high temperature shows increasing viscosity and hardening uponcooling. Therefore, at low speed, there is a defect in that the adhesiveapplied onto the corrugated medium is hardened before being combinedwith linerboard, and that adhesion cannot be carried out.

Some set-back type starch adhesives containing a high content of amylosehave an excellent setting property, but these adhesives have a defectthat once adhesive is hardened upon cooling, it can not become fluideven if it is heated again. Then, such adhesives have a great problem inmachinabilities such as conveying through pipes, applying, storing andthe like, and can not be used in practical application.

On the other hand, Japanese Patent Laid Open Publication No. 105952/1981discloses a method wherein adhesive is applied either onto corrugatingmedium or linerboard, and gelatinizing agents are coated onto the other.Thereafter, contact is carried out each other and adhesion is made bythe reaction of the adhesive and the gelatinizing agents. However, thismethod has a defect that a process is complicated.

Polyvinyl alcohol type adhesives disclosed in Japanese PatentPublication No. 22579/1961, Japanese Patent Laid Open Publication No.3336/1972 and U.S. Pat. No. 3,135,648 are insufficient in green bond,and manufacturing corrugated board at high speed could not be attainedby the method which does not need heating at heat tables afterapplication of adhesive.

Also, heat energy saving could not be accomplished.

OBJECT OF THE INVENTION

Recently, corrugated board production is generally made at the rate of100 m/min or more . Further, from the view of productivity, corrugatedboard is slit and scored on a slitter scorer and cut lengthwise on arotary cutoff direct after a double facer.

This means that high stress operates at an earlier stage of adhesion.Therefore, high initial setting ability and high green bond strength arerequired as characteristics of adhesives.

Thus, the development of adhesives for corrugated board is verydifficult.

When production speed is over 100 m/min, the period of time betweenadhesive application on a glue machine and slitting/scoring on a slitterscorer is only 4 10 seconds. Therefore, in such a short period of time,green bond strength which can stand a stress from a slitter scorer or arotary cutoff is required.

In this point, adhesives for corrugated board are different from thosefor laminated paperboard, paper tube and so on. Furthermore, it isrequired that application amount of adhesive is controlled under theremarkably high shear rate and then an adhesive has remarkably excellentfluidity. On the other hand, the production speed of corrugater ischanged frequently and combining is often carried out at low productionspeed because an operation for paper change and/or order change is oftencarried out.

That is, on high production speed, it is required that an adhesive has aremarkably high setting ability and green bond. On low production speed,adhesive is required having a long open-time because it takes a longtime from application of adhesive at the glue machine to entering intocanvas-covered tables. Thus, there has not been developed a process ofmanufacturing corrugated board having an excellent effect on heat energysaving characterized in that said corrugated board can be manufacturedat high speed without heating after application of water-base adhesive.

SUMMARY OF THE INVENTION

The present inventors have intensively studied an improved process forproduction of corrugated board. As the result, it has been found outthat by using the adhesive containing a specific polyvinyl alcohol andby applicating the adhesive, of which temperature is 60° to 95° C., ontothe flute tips of corrugated medium and combining, corrugated board canbe manufactured from low production speed throughout high productionspeed without heating after application of adhesive.

The present invention provides a process for production of a corrugatedboard by using a water-base adhesive which consists essentially ofpolyvinyl alcohol (hereinafter, referred to "PVA") and clay, the ratioof the clay to the PVA being in the range of 200 to 600 parts by weight(hereinafter, referred to "parts"), based on 100 parts by weight of PVA,the solid content of the adhesive being in the range of 25 to 60% byweight (hereinafter, referred to "%") characterized in that

(a) a polyvinyl alcohol concentration in the solution excluding solidother than polyvinyl alcohol from adhesive being in the region boundedby the following formulas:

    Y≧-0.0014 X+9.4                                     (i)

    Y≦-0.0061 X+26.3                                    (ii)

    200≦X≦3500                                   (iii)

wherein Y is defined by the following formula: ##EQU2## P=Polyvinylalcohol content in the adhesive (by weight) W=Water content in theadhesive (by weight).

and X is degree of polymerization of polyvinyl alcohol,

(b) the viscosity of the adhesive by a Brookfield viscometer being from200 to 6000 centipoises at 85° C., and the rate of increase of theviscosity of the adhesive being in the range of 1.5 to 8 times when thetemperature of adhesive is dropped from 85° to 30° C.,

(c) the adhesive, of which temperature is 60° to 95° C., being appliedonto the flute tips of corrugated medium to manufacture corrugatedboard,

whereby the corrugated board can be satisfactorily produced in a highproduction speed without heating after application of the adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between the degree of polymerization andthe PVA concentration in the solution excluding solid other than PVAfrom an adhesive used in the present invention, wherein X representsdegree of polymerization of PVA and Y represents the concentration ofPVA as defined by following formula : ##EQU3## P=Polyvinyl alcoholcontent in the adhesive (by weight) W=Water content in the adhesive (byweight).

FIG. 2 shows the relationship between a Hercules viscosity at 60°-95° C.under the shear rate of 3×10⁴ sec⁻¹ and the viscosity measured by aBrookfield viscometer, wherein A represents Hercules viscosity and Brepresents the viscosity measured by a Brookfield viscometer.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, when an amount of clay is less than200 parts to 100 parts of PVA, green bond and initial setting abilityare extremely bad. On the other hand, if it exceeds 600 parts, it isinappropriate because it induces excessively less final adhesionstrength and high solid content of the adhesive to give excessively highadhesive viscosity. Therefore, it is preferably in the range of 200 to600 parts based on 100 parts of PVA, more preferably 200 to 450 parts.

The solid content of the water-base adhesive is preferably in the rangeof 25 to 60%. When the solid content of the adhesive is less than 25%,the green bond is too weak. On the other hand, if it exceeds 60%,handling on a corrugator is bad and final adhesion strength is too weak.

Further, the viscosity of adhesive also affects adhesion. It ispreferable that the viscosity of the adhesive at 85° C. by a Brookfieldviscometer is in the range of 200 to 6000 centipoises, more preferably300 to 3000 centipoises.

Furthermore, it is more preferable that the viscosity of appliedadhesive, of which temperature is 60 to 95° C., is in the range of 300to 4000 centipoises. The higher the viscosity, the better its green bondis, but final adhesion strength becomes worse. On the other hand, thelower the viscosity is, the higher final adhesion strength is, but itsgreen bond is insufficient.

It is desirable that the degree of polymerization of PVA used in thepresent invention is in the range of 200 to 3500. When the degree ofpolymerization is less than 200, final adhesion strength is extremelylow. On the other hand, when the degree of polymerization is greaterthan 3500, final adhesion strength becomes lower too, because theviscosity of PVA solution itself is much higher and PVA can not beincorporated into the adhesive in a high content to satisfy initialsetting ability. It is finally preferable that the degree ofpolymerization is in the range of 200 to 3500, more preferably 300 to2500.

Moreover, in order to produce a corrugated board at high productionspeed without heating after applying the adhesive of 60° to 95° C., ithas been found out that PVA concentration in the solution excludingsolid other than PVA from adhesive to degree of polymerization of PVA isconsiderably important. That is, within the degree of polymerization of200 to 3500, PVA having relative low degree of polymerization isavailable in wide range of PVA concentration. However, when PVAconcentration of the adhesive is excessively high, the adhesive hasremarkably high water retention and a decreased green bond. On the otherhand, when PVA concentration is excessively low, the tack of adhesiveand initial setting ability becomes worse and final adhesion strengthbecomes lower. In order to maintain the quality of the corrugated board,the PVA concentration must be relatively high.

Contrarily, when the degree of polymerization is relatively high,viscosity of PVA solution itself becomes high. Then, higher PVAconcentration induces extremely high viscosity of adhesive, high waterretention and less initial setting ability. Therefore, it is requiredthat the PVA concentration is relatively low. However, when the PVAconcentration becomes excessively low, tack of adhesive is insufficientand initial setting ability is inferior.

PVA having a relative high degree of polymerization is available in anarrow range of the PVA concentration in the adhesive. Such PVA tends tobe required that the PVA concentration must be lower than that of PVAhaving low degree of polymerization to decrease the viscosity ofadhesive and the solid content of adhesive must be maintained within asomewhat high range by a high content of clay. However, when theadhesive has an excessively high content of clay and an excessively lowcontent of PVA, it naturally induces a remarkably low final adhesionstrength.

Thus, it has been found out that PVA concentration to degree ofpolymerization of PVA must be in a very limited range in order toproduce corrugated board at high production speed without heating afterapplication of adhesive of which the temperature is 60° to 95° C. As theresults of an intensive study, it has been found out that the adhesivehaving the following formulas (i), (ii) and (iii) is preferable.

    Y≧-0.0014 X+9.4                                     (i)

    Y≦-0.0061 X+26.3                                    (ii)

    200≦X≦3500                                   (iii)

wherein Y is a PVA concentration in aqueous solution excluding the solidother than PVA from adhesive defined as follows: ##EQU4## P=Polyvinylalcohol content in the adhesive (by weight) W=Water content in theadhesive (by weight).

and X is degree of polymerization (viscosity-average degree ofpolymerization defined in JIS K 6726).

That is, the area with a oblique line in FIG. 1 shows a suitable area.

Then, although the degree of hydrolysis of PVA is not limitedspecifically, it is preferably in the range of not less than 70 mol % ,more preferably, not less than 80 mol %. The degree of hydrolysis of PVAis measured by the method defined in JIS K 6726.

There can be used various kinds of PVA such as anion modified PVA,cation modified PVA, acrylamide modified PVA, lactone modified PVA,other various modified PVA produced by copolymerization, after-modifiedPVA (e.g. acetal modified PVA and the like), and modified PVA by othermethod in combination with the above non-modified PVA. Such modified PVAcan be used alone or in combination of two or more.

The clay used in the prsent invention is one or more clay selected fromthe group consisting of kaolinite, halloysite, pyrophyllite andcericite. The clay has preferably an average particle size of not morethan 2 μm for an excellent initial setting ability. However, ifviscosity of an aqueous slurry being prepared by adding the clay intothe water is very high, such clay has less processability, and can notgive a relative high solid content of adhesive and a less improved greenbond. It is preferable that the viscosity of aqueous slurry containingthe clay by a Brookfield viscometer is not more than 1000 centipoises(25° C.) in 25 to 40% of an aqueous dispersion, when the clay is usedalone or in combination with one or more clay selected from the groupconsisting of anionic polymer and metal salts thereof, soluble glass,metal salt of phosphorous compound, and nonion surfactant.

By the way, when the corrugated medium to which adhesive has beenapplied is adhered to the linerboard, the adhering mechanism is asfollows. The water of the adhesive is absorbed by corrugated medium andlinerboard. As a result, adhesive is concentrated and tackified. Thismechanism mainly contributes to the initial setting ability and greenbond of the adhesive in this invention. Further, the viscosity ofadhesive also has a considerable effect upon the green bond, so that itis better than the viscosity of adhesive is higher. On the other hand,as to final adhesion strength, it is deemed that impregnation of theadhesive into the considerable depth of the paper layer of corrugatedmedium and linerboard is required so that the adhesive has preferably alower viscosity.

As mentioned above, in order to produce a corrugated board, it isrequired for sufficient adhesion strength to be obtained within onlyseveral seconds between adhesive application to single-faced board andslitting and scoring. Then, although remarkably excellent green bond isthe most desired characteristic for adhesion of corrugated board, it isalso required that the adhesive has a high final adhesion strength forsecuring the desired strength for final products.

Then, as mentioned above, not only the value of the viscosity ofadhesive is essential, but the adhesive temperature at application stageis essential. That is, when the temperature of adhesive is low atapplication stage, there can not be satisfied both green bond and finaladhesion strength at the same time.

As the results of our various studies with respect to the temperature ofapplied adhesive, it has been found out that when applied adhesiontemperature is 60° to 95° C., both green bond and final adhesionstrength are satisfied at the same time. That is, when the temperatureof applied adhesive of the present invention is less than 60° C., therecan not be satisfied both the green bond and final adhesion strength atthe same time and there can be produced no corrugated board at a highproduction speed, even though the viscosity of adhesive is high.

When the temperature of the adhesive at the application stage is notless than 60° C., there can be obtained sufficient green bond even at ahigh production speed of not less than 140 m/min. There can be alsoobtained a corrugated board having sufficient final adhesion strength.The higher the temperature is at application stage, the bettern theadhesion is. On the other hand, too high temperature induces anexcessive evaporation of water from the adhesive, which makes thecontrol of the concentration of adhesive difficult, and gives film tothe surface of the adhesive together with less machinability. As theresults, the upper limit of adhesive temperature at application stage is95° C. It is finally preferable that the temperature of adhesive at theapplication stage is in the range of 60° to 95° C., more preferably 70°to 90° C.

The initial setting ability and green bond of the adhesive in thepresent invention is improved extremely because by increasing thetemperature of said adhesive the surface energy of said adhesive isreduced, the water of said adhesive is penetrated easily to thecorrugated medium and linerboard, and tack and concentration of saidadhesive are increased rapidly.

The solid content of adhesive used in the present invention seems toaffect the adhesion significantly, and higher solid content of theadhesive brings a greater green bond if the water retention of adhesiveis not excessively high.

If the adhesive gelatinizes by decreasing of adhesive temperature untilabout the room temperature, it induces less processability of theadhesive during conveying through a pipe, storage or the like. Further,is also induces a defect of less adhesion because when in low productionspeed, the adhesive applied onto a single faced corrugated board haslong open time, especially in a double facer part, so that thetemperature of applied adhesive is decreased. Then, the adhesivegelatinizes or the viscosity of the adhesive is extremely increasedbefore the contact to a liner to be laminated.

Therefore, such defect is not induced by the viscosity of the adhesivewhich is increased in the range of 1.5 to 8 times when the temperatureof adhesive is dropped from 85° to 30° C.

In order to adjust the increasing of the viscosity of the adhesive inthe range of 1.5 to 8 times when the temperature of adhesive is droppedfrom 85° to 30° C., it is preferably to use one or more aforementionedclay selected from the group consisting of kaolinite, halloysite,pyrophyllite and cericite. As mentioned below, when a water-solubleboron compound is used, the ratio of increase of the viscosity of theadhesive can be adjusted in the range of 1.5 to 8 times under thespecific conditions such as using of the aforementioned specific clays,operation in neutral to acidic condition, and the like. An adhesive,which contains a PVA and considerable amount of gelatinizing agents suchas Congo red, resorcinol or the like, or contains a PVA, considerableamount of water-soluble boron compound and an alkali filler, can notadjust the rate of increase of viscosity within the range of 1.5 to 8times.

Moreover, at present a corrugating machine is operated at a highproduction speed of 100 m/min or more, and the control of applicationamount of adhesive in a glue machine is carried out under a considerablehigh shear rate of not less than 1×10⁴ sec⁻¹ so that the fluidcharacteristics of adhesive under high shear rate seem to affect theadhesion significantly.

As the results of our studies, it has been found out that the viscosity(A) at 60° to 95° C. of the adhesive by a Hercules viscometer under theshear rate of 3×10⁴ sec⁻¹ and the viscosity (B) at the same temperatureby a Brookfield viscometer have a relation of the following formula (iv)##EQU5## and the viscosity of the adhesive by a Brookfield viscometer isin the range of 300 to 3000 centipoises at 85° C. Although there is nolower limit of A /√B, it is usually not less than 1. This relation isshown in FIG. 2, which illustrates that adhesion of adhesive having lessratio (formula iv) of the viscosity under high shear rate to that underlower shear rate is greater than that of adhesive having a higher ratio.

In this connection, Japanese Patent Laid Open Publication No. 3336/1972exemplifies an adhesive showing pseudoplasticity under a shear rate of1×10² to 1×10⁴ sec⁻¹, which relates to specific PVA (the viscosity of 4%aqueous solution is in the range of 9 to 16 centipoises) at atemperature of not more than 50° C.

On the other hand, the present invention relates to the adhesiveemploying PVA having a wide range of degree of polymerization of200-3,500. Further, with respect to the viscosity measured by aBrookfield viscometer at 85° C. being 300-3,000 centipoises, it has beenfound that the adhesives, which have the ratio of viscosity at a highshear rate of 3×10⁴ sec⁻¹ (Hercules viscosity) against the square rootof the viscosity measured at a low shear rate being lower than thespecific value, are especially excellent as the adhesives for theproduction of the corrugated board. Furthermore, we have found that theadhesive temperature of application stage, as mentioned above,considerably affects initial setting ability. That is, when thetemperature is not higher than 50° C., the initial setting ability at ahigh production speed is bad. It has been also found that the adhesives,which have a lower ratio of Hercules viscosity at a high shear rateagainst the viscosity at a low shear rate with the increasing of thetemperature, are excellent in adhesion property.

As illustrated above, the water-base adhesive containing PVA and clay asmain ingredients can be used for production of a corrugated board in aheat-saving and high-speed production, however, water-soluble boroncompound can be also used in combination with PVA and clay.

As the water-soluble boron compound, boric acid, borate such as borax,and borate ester of polyhydric alcohol such as glycerol and ethyleneglycol can be used, and among them boric acid is suitable.

It is preferable to use the water-soluble boron compound in an amount ofnot more than 15 parts, based on 100 parts of PVA. The lower bound isnot specially limited, however, it is desirably to use in an amount ofnot less than 0.1 parts. The more used, the better green bond isprovided because of increased tackiness of adhesives. In this case,however, it is not desirable that the adhesives tend to be gelatinizedat a low temperature. Moreover, by decreasing of the adhesivetemperature from 85° to 30° C., the ratio of increase of the viscositybecomes more than 8 times to induce a worse processability. It is morepreferable to use a relatively greater amount of water-soluble boroncompound against the PVA having a low degree of polymerization. On theother hand, it is preferable to use less amount of such compound againstthe PVA having a high degree of polymerization. It is preferable thatthe viscosity of the solution, excluding solid other than PVA andwater-soluble boron compound from the adhesive, measured at 60°-95° C.by a Brookfield viscometer is in the range of 100-1,000 centipoises. Theaddition of the following peroxide into the adhesives improves theadhesion, however, in this case the viscosity is decreased. Therefore,the viscosity of the solution, excluding solid other than PVA andwater-soluble boron compound from the adhesive, measured at 60°-95° C.by Brookfield viscometer is suitably in the range of 150-1,500centipoises.

When the relatively greater amount of water-soluble boron compound isused, attention should be paid to the pH of the adhesive. That is, whenthe adhesive is alkaline, the bonding of PVA with water-soluble boroncompound is facilitated and the adhesive is easily gelatinized at a lowtemperature. Then, the ratio of increase of viscosity can not be in therange of 1.5-8 times, when the temperature of adhesive is dropped from85° to 30° C., which results in less handling of adhesives and lessimproved adhesion. Therefore, pH of the adhesives is preferably in theregion from neutral to acidic conditions, preferably not higher than 6.

The adhesives for the production of a corrugated board having suchproperties may include those containing only PVA and clay, or PVA, clayand water-soluble boron compound. In order to improve its fluidity andadhesion property, however, it is more preferably to add to themmetallic salt of phosphoric acid compound, such as sodium polyphosphate,sodium hexamethaphosphate and the like, dispersing agent for clay suchas water glass and the like; anionic polymeric compound and metallicsalt thereof such as polyacrylic acid and a salt thereof, sodiumalginate, isobutene-maleic anhydride copolymer; nonionic surface activeagent such as higher alcohol added with ethylene oxide, copolymer ofethylene oxide and propylene oxide and the like; or guar gum. Among theabove said fluid additives, sodium hexamethaphosphate, sodiumpolyacrylate or sodium alginate is preferred. It is preferably added tothe adhesive in an amount of 0.01-2 parts, based on 100 parts of clay.When more than 2 parts, the initial setting ability becomes bad. On theother hand, when less than 0.01 parts, there can be found no effect.

Moreover, it is effective for improvement of adhesion property to addperoxides such as sodium periodate, periodic acid, potassium periodate,sodium persulfate, potassium persulfate, ammonium persulfate and thelike. Among them, periodic acid, sodium periodate and sodium persulfateare preferable. The addition of peroxide will decrease the viscosity tofacilitate handling and also improve tackiness of adhesives as well asinitial setting ability. However, incorporation of excessive amount ofperoxide excessively decreases the viscosity and the initial settingability becomes bad. Therefore, it is preferably to incorporate in anamount of 0.01-10 parts of peroxide based on 100 parts of PVA. Additionof peroxide of less than 0.01 parts may induce little effective.

Further, the addition of saccharose, colloidal silica, calciumcarbonate, carbon black, white carbon or silica anhydride improvestackiness and adhesion property. It is preferable to incorporate in anamount of 0.1-20 parts, based on 100 parts of clay, more preferably0.5-15 parts. The amount of less than 0.1 parts induces lesseffectiveness, and amount of more than 20 parts induces an adverseeffect on fluidity, and such amount cannot be used. Even if the additionof large amount of white carbon or calcium carbonate results in badadhesion, the addition of a small amount of clay improves adhesionproperty.

As mentioned above, it is preferably that the degree of polymerizationof PVA is 200-3,500, more preferably 300-2,500. It is preferable thatdegree of hydrolysis of PVA is not less than 70 mol %, more preferablynot less than 80 mol %. The PVA can be used in combination with one ormore grades thereof. As the results of investigations about combinationof various PVA grades, it has been found that the adhesion having aspecific combination of more than two grades of PVA shows a furtherimproved adhesion property compared to that of adhesion employing onlyone grade of PVA, even if they have the same average degree ofpolymerization degree and the same average degree of hydrolysis.

That is, regarding the degree of polymerization, the mixed PVA whichcontains 10-90 parts of PVA having the degree of polymerization of200-800, preferably 300-600, and 90-10 parts of PVA having degree ofpolymerization of 1,100-3,500, preferably 1300-3500, more preferably1,700 -2,500 shows further improved adhesion property compared to thatobtained from only one grade of PVA having the same average degree ofpolymerization (said PVA is polymerized to be adjusted to the desireddegree of polymerization and hydrolysis per se), while both of mixed PVAand one grade of PVA have almost the same viscosity. When either PVAhaving degree of polymerization of 200-800 or PVA having degree ofpolymerization of 1100-3500 is used in an amount of less than 10 partsbased on 100 parts of the whole amount of PVA, the adhesive shows nodifferent adhesion property from that obtained by using of only onegrade of PVA. Although the reason is not clear, it seems that the degreeof polymerization of mixed PVA consisting of two or more grades of PVAhaving different degrees of polymerization is widely distributedcompared with that of only one grade of PVA. That is, the mixed PVA hasa PVA having relatively lower degree of polymerization and a PVA havingrelatively higher degree of polymerization compared to the only onegrade of PVA, even if they have the same average degree of thepolymerization. In the adhesive containing such mixed PVA, PVA havingrelatively lower degree of polymerization facilitates the penetration ofan adhesive solution into paper layer and improves adhesive propertybecause clay is well dispersed by that PVA, while PVA having relativelyhigher degree of polymerization contributes to improve tackiness whichsignificantly affect a green bond of the adhesive.

As to degree of hydrolysis, it has been found out that the adhesivecontaining mixed PVA which consisting of 10-90 parts of PVA (a) havingdegree of hydrolysis of 72-100 mol % and 90-10 parts of PVA (b) havingdegree of hydrolysis lower by 2-30 mol % than that of PVA (a) issuperior in adhesion property to the adhesive containing the PVA whichconsists of only one grade of PVA (PVA prepared to be adjusted todesired degree of hydrolysis per se) having the same degree ofhydrolysis.

Although the reason for further improvement of adhesion property is notclear, it seems that the degree of hydrolysis of mixed PVA consisting oftwo or more grades of PVA having different degrees of hydrolysis iswidely distributed compared with that of only one grade of PVA. That is,the mixed PVA has a PVA having relatively lower degree of hydrolysis anda PVA having relatively higher degree of hydrolysis compared with thatof the only one grade of PVA, even if they have the same average degreeof hydrolysis. Generally, PVA having relatively lower degree ofhydrolysis shows relatively lower surface energy when dissolved in waterand can easily penetrate into the paper layer, while PVA havingrelatively higher degree of hydrolysis contributes to the green bondowing to its excellent wet tack of the adhesive containing that PVA, sothat adhesive containing such mixed PVA is improved in both green bondstrength and final adhesion strength. However, it should be noted thatthe adhesive containing the PVA which contains greater amount of PVAhaving higher degree of hydrolysis and higher degree of polymerizationshows higher viscosity, so that said adhesive may hardly be adjusted tohigh solid content. While, it should be noted that the adhesivecontaining the PVA which contains greater amount of PVA having lowerdegree of hydrolysis and lower degree of polymerization especially showsless wet tack.

When using more than two grades of PVA, there may be many possibilitiesfor the combination of PVA having different degrees of polymerizationand/or hydrolysis, such as a combination of more than two grades of PVAhaving the same degree of polymerization but different degree ofhydrolysis, a combination of those having the same degree of hydrolysisbut different degree of polymerization, a combination of PVA havinglower degree of polymerization and higher degree of hydrolysis with PVAhaving higher degree of polymerization and lower degree of hydrolysis,and vice versa. The adhesive employing the PVA of above combination incertain ratio shows an further improved adhesion property compared withthose employing only one grade of PVA. Among them, the adhesivecontaining the combination of PVA which comprises completely hydrolyzedPVA having lower degree of polymerization and partially hydrolyzed PVAhaving higher degree of polymerization, and containing boric acid showsa remarkably excellent adhesion property. Although the PVA which ishydrolyzed in the conventional methanol solution is effective, PVA whichis hydrolyzed in methyl acetate or other solvent, or the mixture ofmethanol and other solvent can be also used.

The adhesives in the present invention can be used for paper, such aslaminating paper; board and corrugated board, paper tube, paper bag andthe like, in addition to a corrugated board application. In the aboveusage, it is not necessary to conduct an adhesion in a short time asthat for a corrugated board. However, recently, an improvement ininitial setting ability has been required with the increasing ofproduction speed. Therefore, the present invention can employadvantageously such a high production speed.

In order to prepare the present adhesive, the mixture of PVA, clay,water-soluble boron compound or other additives previously prepared maybe firstly poured into water with stirring to prepare a slurry, or ametallic salt of phosphoric acid compound, dispersant for clay such aswater-glass, additives such as anionic polymeric compound and a metallicsalt thereof and nonionic surface-active agent, clay, PVA, water-solubleboron compound or other additives may be poured into water one afteranother with stirring to prepare slurry. The process for preparing theadhesive may be conducted whether in a batch process or in a continuousprocess, and it may be prepared by heating and dissolving by any heatingprocess which comprises, for example directly blowing a steam vapor intothe slurry solution in a jet-cooker or a preparation bath, or indirectlyheating a preparation bath or pipe.

Peroxide may be added either before heating and dissolving, or afterheating at high temperature. However, it is preferably to add it intothe slurry solution just before heating for efficiency.

In the process having an excellent effect on heat energy saving and canbe manufactured at high production speed of a corrugated board of thepresent invention, the production can be carried out by pre-heating oflinerboard or single faced corrugated boards, or by heating of heattables.

According to the process for production of a corrugated board of thepresent invention, it becomes possible to produce at high speed of 100m/min or more, and if desired, 120 m/min or more. It is also possible toproduce at a remarkably high speed of up to 140 m/min.

The process for production of a corrugated board of the presentinvention has a great advantage that it can produce corrugated board ata wide range of production speed, from low to high, without heatingafter application of the adhesive and laminating, and it also has anexcellent effect on heat energy saving.

Moreover, the process for production of a corrugated board of thepresent invention has a great effect to reduce warpage of the corrugatedboard sheets.

The present invention is further illustrated by the following tabletests and examples in detail but should not be construed to be limitedthereto.

A. PVA and clay used are as follows:

(1) PVA

    ______________________________________                                                         Degree of                                                         Degree of   Hydrolysis                                                   No.  Polymerization                                                                            (mol %)                                                      ______________________________________                                        A      550       98.6       (Kuraray Poval PVA-105)                           B      700       98.3                                                         C    1,100       98.7       (Kuraray Poval PVA-110)                           D    1,750       98.5       (Kuraray Poval PVA-117)                           E    1,750       88.5       (Kuraray Poval PVA-217)                           F    2,450       98.5       (Kuraray Poval PVA-124)                           G    1,500       98.2                                                         H      330       98.6                                                         I      550       88.4       (Kuraray Poval PVA-205)                           J    2,450       95.2       (Kuraray Poval PVA-624)                           K    1,750       96.1       (Kuraray Poval PVA-CST)                           L    1,300       93.6       (Kuraray Poval PVA-613)                           M    2,450       88.2       (Kuraray Poval PVA-224)                           N    3,300       88.3                                                         ______________________________________                                    

(2) Clay

I: UG-90 (ENGELHARD): Average Particle Size, 0.3 μm, Kaolinite

II: ASP-400 (ENGELHARD): Average Particle Size, 4.8 μm, Kaolinite

III: KANATANI-CLAY NSF (KANATANI KOGYO) Particle Size less than 2 μm,99%, Halloysite:

B. Evaluation of green bond and initial setting ability, evaluation ofsuitability for laminating at a high or low production speed,measurement of adhesive strength, viscosity were carried out as follows:

(1) Evaluation of green bond and initial setting ability according totable test

An applicator roll controlled by a given temperature was rotated in anadhesive controlled at a given temperature. The amount of adhesives onroll was controlled with a doctor blade, thereafter flute tips of thesingle faced corrugated board was lightly pressed onto the roller toapply adhesive thereon. Then, immediately, it was laminated to a liner,and thereafter pinched lightly for 3-10 seconds. Then, the resultingproducts were subjected to forced peeling test and the condition ofadhesion was observed ( ⊚: Excellent, ○: Good, Δ: Not Bad, x: bad, xx:Very Bad). As single faced corrugated board and linerboard, 220 g/m² ofJIS B-Grade linerboard and 125 g/m² of JIS B-Grade corrugating mediumwere used, respectively.

(2) Evaluation with Corrugator

The corrugator used is 1600 mm wide and has a maximum speed of 140m/min. The conventional heating on the double facer was not carried out.A linerboard (JIS B-Grade; 220 g/m²) and a corrugating medium (JISB-Grade; 125 g/m²) were mainly used.

The adhesive of the present invention, which was prepared as mentionedabove, was supplied to a glue machine of which the glue unit was held to60°-95° C., was transferred and applied onto flute tips of a singlefaced corrugated board with an application roll to adhere saidsingle-faced board with linerboard. The resulting corrugated board waspassed through slitter-scorer with corrugator to slit and score, andfurther rotary cutoff and was cut on a rotary cutoff.

(a) Evaluation of adhesive suitability for production at a high speedwith a corrugator

Adhesive suitability was evaluated by Highest Combining Speedrepresenting the highest speed that corrugated board is combined atwithout any adhesion failure in slit and/or scored parts of thecorrugated blank. For finding Highest Combining Speed, corrugated boardmade on the corrugator was immediately delaminated at the end of arotary cutoff, and was checked whether there was said adhesion failure.

(b) Evaluation of adhesive suitability for production at a low speedwith a corrugator

The corrugator was operated at the lowest speed of 25 m/min and theadhering condition was examined at the end of a rotary cutoff on thecorrugator. Low Speed Evaluation means the evaluation of adheringcondition of corrugated board manufactured at 25 m/min.

(3) Method for adhesive strength

Adhesive strength was measured with single adhesion separating testspecified in JIS Z-0402.

(4) Measurement of Viscosity

(a) Brookfield viscosity (centipoise)

Brookfield viscometer, No. 1-No. 4 rotor, 30-60 rpm.

(b) Viscosity under high shear rate (centipoise)

Hercules high speed viscometer (produced by KUMAGAI RIKI KOGYO CO.,LTD.)

Bob B; highest rotating speed, 8800 rpm; sweep time 10 sec.; Herculesviscosity (η) is defined by the following equation: ##EQU6## T: torque(dyne:cm) S: shape factor =0.0004

N: rotation=7,200 rpm (shear rate; 3×10⁴ sec⁻¹)

The terms "Parts" and "%" used herein mean parts by weight and weight %,respectively.

EXAMPLES 1 TO 29

The mixtures having the formula shown in Table 1 were heated and stirredfor one hour and dissolved to prepare adhesive. The green bond andinitial setting ability and the result of the adhesive evaluation testwith a corrugator of these adhesives are shown in Table 2. In theadhesive evaluation test, "A" flute was used and the application amountrepresents the amount of solid content of an adhesive applied ontosingle-face side of corrugated board.

As is clear from Examples 1-29, it has been found that an excellentadhesion can be obtained by applying said 60°-95° C. adhesive having aratio of PVA concentration against polymerization degree thereof withinan area shown by an oblique line of the FIG. 1, and having a solidcontent of 25-60% and a viscosity being within a range of 200 to 6,000centipoise at 85° C., and the viscosity of the adhesive being increasednot more than 8 times when the adhesive is cooled from 85° C. to 30° C.

As is clear from Comparative Example 2, an adhesive having excessivelyhigh viscosity at 85° C. has a inferior adhesion. As is clear fromComparative Example 6, an adhesive easily to be gelatinized is inferiorin laminating ability at a low production speed. Further, thetemperature of the adhesive at the application stage gives a greateffect on green bond, initial setting ability and suitability forcombining at a high production speed. That is, the temperature not lowerthan 60° C. is suitable (see Examples 1-29), and when the temperature ofadhesive at application stage is below 60° C. (see Comparative Examples3 and 4), the green bond and initial setting ability and suitability forcombining at a high production speed were insufficient.

As is clear from Examples 7-29, the adhesives, which have a viscosity ofan aqueous PVA-boric acid solution at temperature of 60°-95° C. in arange of 100 to 1,500 centipoises, shows a good adhesion property.

As is clear from Examples 10-20, the adhesion is further improved byadding anionic polymer compound, phosphoric acid compound, peroxide,colloidal silica (Snowtex C, NISSAN CHEMICAL CO., LTD.), white carbon(Silton R-2, Mizusawa Kagaku kk), Carbon black (#44, Mitsubishi KaseiKK), calcium carbonate, silica anhydride (AEROSIL 380, AEROSIL NIPPON)or guar gum (MAYPROGUM SN-20, SANSHO KK).

In Example 19, an adhesive is prepared in the same manner andcompounding ratio as described in Example 10 except that the mixture ofPVA (A) and (F) was substituted for PVA (C), wherein an average degreeof polymerization of the PVA mixture was adjusted to be the same as thatof PVA (C). The adhesive of Example 19 shows almost the same viscosityas that of example 10, but it also shows a further improved adhesion.Example 21 or 23 contained one grade of PVA (K) or PVA (L) respectively,and Example 22 contained two grades of PVA (D) and (E).

An average degree of polymerization and an average degree of hydrolysisof mixed PVA which comprises PVA (D) and (E) are nearly equal to thoseof PVA (K).

Example 24 comprises three grades of PVA (A), (I) and (M). Example 25comprises three grades of PVA (A), (I) and (F). An average degree ofpolymerization and an average degree of hydrolysis of mixed PVA whichcomprises PVA (A), (I) and (M), or PVA (A), (I) and (F), are nearlyequal to those of PVA (L).

In the comparison of example 21 with 22, or 23 with 24 and 25, it hasbeen found that the adhesive comprising two grades of PVA or more showsa further improved adhesion compared to that of one grade of PVA.Further, in the comparison of examples 24 and 25, it is obvious thatadhesive of example 24 using completely hydrolyzed PVA having low degreeof polymerization and partially hydrolyzed PVA having high degree ofpolymerization, shows a better adhesion property. As is obvious fromExamples 24, 25, 26, 27, and 29, each adhesive, which contains twogrades or more of PVA having different degrees of polymerization andhydrolysis in a specified ratio, shows a remarkably excellent adhesionproperty.

The viscosity of 40% slurry solution of clay (I) at 25° C. measured withBrookfield viscometer was 10 centipoises and that of 32% slurry solutionof clay (III) was 800 centipoises.

                                      TABLE 1                                     __________________________________________________________________________    Formula (parts)                                                                                         Boric              Solid PVA*                       No. PVA (1)                                                                            PVA (2)                                                                            PVA (3)                                                                            Clay   Acid                                                                              Additives  Water                                                                             Conc (%)                                                                            Conc                       __________________________________________________________________________                                                       (%)                        Ex                                                                             1  A 14.6         III 29.4                                                                             --  --      -- 56.0                                                                              44.0  22.6                        2  D 11.7         III 23.4                                                                             --  --      -- 64.9                                                                              35.1  15.3                        3  F 6.0          III 24.1                                                                             --  --      -- 69.9                                                                              30.1  7.9                         4  H 12.3         III 36.6                                                                             0.64                                                                              --      -- 50.5                                                                              49.5  19.6                        5  F 8.3          I   22.5                                                                             0.42                                                                              --      -- 68.8                                                                              31.2  10.8                        6  A 5.9          I   32.7                                                                             0.87                                                                              --      -- 60.5                                                                              39.5  8.9                         7  F 4.7          III 26.0                                                                             0.16                                                                              --      -- 69.2                                                                              30.8  6.4                         8  D 5.6          I   25.8                                                                             0.54                                                                              --      -- 68.0                                                                              32.0  7.6                         9  E 7.8          I   21.8                                                                             0.70                                                                              --      -- 69.2                                                                              31.0  10.2                       10  C 8.6          I   25.2                                                                             0.60                                                                              Sodium  0.1                                                                              65.5                                                                              34.5  11.6                                                     Polyacrylate                                    11  D 6.1          I   23.3                                                                             0.90                                                                              Sodium  0.1                                                                              69.4                                                                              30.6  8.1                                                      Alginate                                                                      Sodium  0.2                                                                   Periodate                                       12  D 7.7          I   22.1                                                                             0.80                                                                              Sodium  0.2                                                                              69.3                                                                              30.7  10.0                                                     Persulfate                                      13  G 8.1          III 23.2                                                                             0.80                                                                              Sodium  0.4                                                                              67.5                                                                              32.5  10.7                                                     Hexametha-                                                                    phosphate                                       14  D 5.6          I   24.8                                                                             0.54                                                                              Colloidal                                                                             1.0                                                                              68.0                                                                              32.0  7.6                                                      Silica                                          15  D 5.6          I   24.8                                                                             0.54                                                                              Carbon Black                                                                          1.0                                                                              68.0                                                                              32.0  7.6                        16  D 5.6          I   24.8                                                                             0.54                                                                              White Carbon                                                                          1.0                                                                              68.0                                                                              32.0  7.6                        17  D 5.6          I   24.8                                                                             0.54                                                                              Calcium 1.0                                                                              68.0                                                                              32.0  7.6                                                      Carbonate                                       18  D 5.6          I   24.8                                                                             0.54                                                                              Silica  1.0                                                                              68.0                                                                              32.0  7.6                                                      Anhydride                                       19  A 3.9                                                                              F 1.7     I   25.2                                                                             0.60                                                                              Sodium  0.1                                                                              65.5                                                                              34.5  11.6                                                     Polyacrylate                                    20  A 3.9                                                                              F 1.7     I   25.2                                                                             0.60                                                                              Guar Gum                                                                              0.1                                                                              65.5                                                                              34.5  11.6                       21  K 8.1          I   18.2                                                                             0.65                                                                              --      -- 73.1                                                                              26.9  10.0                       22  D 6.2                                                                              E 1.9     I   18.2                                                                             0.65                                                                              --      -- 73.1                                                                              26.9  10.0                       23  L 9.5          I   21.4                                                                             0.76                                                                              --      -- 68.4                                                                              31.6  12.2                       24  A 4.8                                                                              I 0.8                                                                              M 3.9                                                                              I   21.4                                                                             0.76                                                                              --      -- 68.4                                                                              31.6  12.2                       25  A 0.9                                                                              I 4.7                                                                              F 3.9                                                                              I   21.4                                                                             0.76                                                                              --      -- 68.4                                                                              31.6  12.2                       26  A 9.5                                                                              N 1.1     I   23.9                                                                             0.85                                                                              --      -- 64.7                                                                              35.3  14.1                       27  H 0.76                                                                             D 6.7     I   21.0                                                                             0.67                                                                              --      -- 70.8                                                                              29.2  9.6                        28  H 0.37                                                                             D 6.9     I   20.3                                                                             0.65                                                                              --      -- 71.8                                                                              28.2  9.2                        29  A 5.6                                                                              J 3.7     I   20.9                                                                             0.74                                                                              --      -- 69.1                                                                              30.9  11.9                       Comp.                                                                         Ex                                                                             1  B 6.2          III 26.7                                                                             0.19                                                                              --      -- 72.0                                                                              28.0  7.9                         2  D 11.8         III 26.7                                                                             0.19                                                                              --      -- 61.3                                                                              38.7  16.1                        3  F 4.7          III 26.0                                                                             0.16                                                                              --      -- 69.2                                                                              30.8  6.4                         4  D 7.7          I   22.1                                                                             0.80                                                                              Sodium  0.2                                                                              69.3                                                                              30.7  10.0                                                     Persulfate                                       5  F 3.7          I   20.6                                                                             0.37                                                                              Sodium   0.12                                                                            75.2                                                                              24.8  4.7                                                      Polyacrylate                                     6  D 5.9          CaCO.sub.3                                                                        13.6                                                                             0.16                                                                              --      -- 80.4                                                                              19.6  6.8                        __________________________________________________________________________     *PVA concentration in the solution excluding solid other than PVA from        adhesive.                                                                

                                      TABLE 2                                     __________________________________________________________________________                          Application Condition                                                                         Adhesion                                                      Viscosity            Corrugator                         Viscosity of      Adhe-                                                                             of PVA-    Appli-*                                                                            Table      Corruga-                                                                           Final                   Adhesive (cP)     sive                                                                              Boric Acid                                                                          Viscosity                                                                          cation                                                                             Test Highest***                                                                          tion at                                                                            Adhesion                    85° C.                                                                     30° C.                                                                           Temp.                                                                             Solution                                                                            of Adhe-                                                                           Amount                                                                             Green**                                                                            Combining                                                                           Low****                                                                            Strength                                                                           ******             No. (a) (b)   (b)/(a)                                                                           (°C.)                                                                      (cp)  sive (cp)                                                                          (g/m.sup.2)                                                                        Bond Speed Speed                                                                              *****                                                                              A/√B        __________________________________________________________________________    Ex                                                                             1  440 2,200 5.0 60  --    850  10.1 ○                                                                            100  Good 20.1 6.5                 2  4,300                                                                             22,800                                                                              5.3 90  --    3,700                                                                              7.3  ○                                                                            100  Good 21.2 6.7                 3  330 1,800 5.5 80  --    350  8.3  ○                                                                            100  Good 20.8 6.8                 4  750 2,030 5.0 75   50   790  12.5 ○                                                                             100 Good 20.3 6.6                 5  5,700                                                                             45,000                                                                              7.9 90  1,600 4,900                                                                              8.0  ○                                                                            100  Good 20.1 7.1                 6  270   560 2.6 90   25   255  10.5 ○                                                                            100  Good 20.2 6.6                 7  570 2,800 4.9 80  110   650  7.2   ○ -⊚                                                         120  Good 22.1 6.2                 8  390 3,040 7.8 70  200   640  6.3   ○ -⊚                                                         120  Good 20.5 6.1                 9  2,000                                                                             7,400 3.7 95  630   1,700                                                                              7.5   ○ -⊚                                                         120  Good 21.8 5.8                10  460 3,350 7.3 80  150   540  7.0  ⊚                                                                   >140  Good 20.5 5.7                11  420 2,340 5.6 70  800   640  7.2  ⊚                                                                   >140  Good 22.3 5.5                12  1,600                                                                             5,200 3.3 75  1,300 1,900                                                                              6.5  ⊚                                                                   >140  Good 21.8 5.5                13  1,010                                                                             3,960 3.9 85  370   1,010                                                                              6.5  ⊚                                                                   >140  Good 22.3 5.8                14  430 3,210 7.5 70  200   700  6.5  ⊚                                                                   >140  Good 20.4 6.2                15  410 3,100 7.6 70  200   670  6.4  ⊚                                                                   >140  Good 20.7 6.1                16  400 3,080 7.7 70  200   660  6.4  ⊚                                                                   >140  Good 20.3 6.1                17  440 3,380 7.7 70  200   750  6.5  ⊚                                                                   >140  Good 20.2 6.1                18  400 3,100 7.8 70  200   670  6.4  ⊚                                                                   >140  Good 20.5 6.1                19  450 3,300 7.3 70  155   740  7.4  ⊚                                                                   >140  Good 21.2 5.7                20  470 3,400 7.2 70  155   760  7.4  ⊚                                                                   >140  Good 21.3 5.7                21  1,020                                                                             5,960 5.8 75  610   1,340                                                                              8.5  ○                                                                            110  Good 21.7 5.7                22  1,030                                                                             5,980 5.8 75  610   1,350                                                                              8.4  ⊚                                                                   >140  Good 21.4 5.6                23  900 5,490 6.1 80  710   1,040                                                                              7.3  ○                                                                            110  Good 20.9 5.8                24  880 5,420 6.2 80  700   1,030                                                                              7.4  ⊚                                                                   >140  Good 21.2 5.7                25  890 5,450 6.1 80  700   1,040                                                                              7.3   ○ -⊚                                                         130  Good 20.5 5.7                26  540 2,950 5.5 65  120   950  8.0  ⊚                                                                   >140  Good 20.4 6.2                27  740 3,950 5.3 90  210   650  7.5  ⊚                                                                   >140  Good 21.3 6.2                28  800 4,150 5.2 90  160   700  7.5   ○ -⊚                                                         120  Good 21.5 6.2                29  1,020                                                                             5,970 5.9 70  600   1,570                                                                              7.0  ⊚                                                                   >140  Good 20.5 5.7                Comp                                                                          Ex                                                                             1   45   160 3.6 90   10    41  8.5  xx   Cannot be                                                                           Bad  --                                                                 Corrugated                          2  8,500                                                                             48,500                                                                              5.7 90  5,400 7,500                                                                              10.5 Δ                                                                            Cannot be                                                                           Bad  --                                                                 Corrugated                          3  570 2,800 4.9 40  350   2,000                                                                              6.0  x      30  Good 22.3                     4  1,600                                                                             5,200 3.3 50  2,800 3,200                                                                              6.5  Δ                                                                              60  Good 22.5                     5   63   270 4.3 85   38    63  6.8  x    Cannot be                                                                           Bad  --                                                                 Corrugated                          6  450 >60,000                                                                             >100                                                                              85  --    450  6.5  Δ                                                                              80  Bad  --                      __________________________________________________________________________     *Application amount of adhesive or one side of corrugating board (solid       base)                                                                         **Green Bond and Initial Setting Ability.                                     ***Corrugatable speed. Highest speed of corrugator is 140 m/min.              ****Degree of adhesion at 25 m/min.                                           *****kg/40 cm.sup.2                                                           ******A: Hercules viscosity, B: Brookfield viscosity                     

EXAMPLE 30

25 Parts of PVA (D), 72.3 parts of clay (I), 2.5 parts of boric acid,and 0.2 parts of sodium alginate were dissolved into water to give anadhesive having a solid concentration of 27.8%. The viscosity ofadhesive measured by Brookfield viscometer was 1,960 centipoises at 65°C. and 820 centipoises at 90° C. Hercules viscosity of the adhesiveunder high shear rate of 3×10⁴ sec⁻¹ was 250 centipoises at 65° C. and155 centipoises at 90° C. The ratio at the same temperature, (HerculesViscosity)/√(Brookfield Viscosity), was 5.6 at 65° C. and 5.4 at 90° C.(see FIG. 2).

A adhesive evaluation test was carried out by using the adhesive of 65°C. The adhesive had a good green bond and a good combining ability athigh production speed, and also had a high final adhesion strength valueof 21.7 kg/40 cm² at an application amount of 6.0 g/m² (solid base).

EXAMPLE 31

24 Parts of PVA (D), 73.6 parts of clay (III), and 2.4 parts of boricacid were dissolved into water to give an adhesive having a solidconcentration of 27.6%. The viscosity of adhesive measured by Brookfieldviscometer was 1,550 centipoises at 65° C. and 360 centipoises at 90° C.Hercules viscosity of the adhesive under high shear rate of 3×10⁴ sec⁻¹was 280 centipoises at 65° C. and 145 centipoises at 90° C. The ratio atthe same temperature, (Hercules Viscosity)/√(Brookfield Viscosity), was7.1 at 65° C. and 7.6 at 90° C., that is, the ratio at 90° C. is higherthan that at 65° C. (see FIG. 2).

An adhesive evaluation test was carried out at 65° C. by using theadhesive. Green bond and combining ability at high production speed ofthis adhesive is inferior to that of Example 30.

EXAMPLE 32

24 Parts of PVA (C), 73.2 parts of clay (I), 2.4 parts of boric acid,0.1 parts of sodium polyacrylate and 0.3 parts of sodium persulfate weredissolved into water to give an adhesive having a solid concentration of35.5%. The viscosity of the adhesive measured by a Brookfield viscometerwas 850 centipoises at 90° C., while Hercules viscosity of the adhesiveat high shear rate (3×10⁴ sec⁻¹) was 156 centipoises at 90° C. Theadhesive had a ratio of 5.4, [(Hercules Viscosity)/√(BrookfieldViscosity)=156/√850 ] (see FIG. 2).

A adhesive evaluation test was carried out at 90° C. by using theadhesive. The adhesive had a good green bond and a good suitability forcombining at a production speed of not less than 140 m/min, and also hada high adhesion strength of 21.5 kg/40 cm² at an application amount of6.5 g/m² (solid base).

EXAMPLE 33

24 Parts of PVA (B), 73.3 parts of UG-90 (average particle size: 0.3μm), 2.4 parts of boric acid and 0.2 parts of sodium polyacrylate weredissolved into water to give an adhesive having a solid concentration of37.0%. The viscosity by a Brookfield viscometer of the adhesive was 980centipoises at 85° C. and 3,510 centipoises at 30° C. Further, theadhesive having a solid concentration of 42.4% prepared in the samemanner as described above except that ASP-400 (average particle size:4.8 μm) was used instead of the said UG-90 had a viscosity by aBrookfield viscometer of the adhesive of 1,000 centipoises at 85° C. and4,300 centipoises at 30° C. In the comparison of the green bond of bothadhesives, both adhesive had a good green bond, however, that of theadhesive using UG-90 was better.

Moreover, the adhesive using white carbon instead of clay had a badgreen bond.

What is claimed is:
 1. A process for production of corrugated boardusing an adhesive which comprises water, polyvinyl alcohol, clay and awater-soluble boron compound, the ratio of clay to polyvinyl alcoholbeing in a range of 200 to 600 parts by weight based on 100 parts byweight of polyvinyl alcohol, the ratio of the water-soluble boroncompound being not more than 15 parts by weight based on 100 parts byweight of polyvinyl alcohol, the solid content of the adhesive being 25to 60% by weight, characterized in that(a) a polyvinyl alcoholconcentration in the solution excluding solid other than polyvinylalcohol from the adhesive being in the region bounded by the followingformulas: Y≧-0.0014 X+9.4 (i) Y≦-0.0061 X+26.4 (ii) 20≦ X≦3500 (iii)wherein Y is defined by the following formula: ##EQU7## P=Polyvinylalcohol content in the adhesive (by weight) W=Water content in theadhesive (by weight) and X is degree of polymerization of the polyvinylalcohol, (b) the viscosity of the adhesive measured by a Brookfieldviscometer being from 200 to 6000 centipoises at 85° C., and the rate ofincrease of the viscosity of the adhesive being in the range of 1.5 to 8times when the temperature of adhesive is dropped from 85° to 30° C.,(c) the clay being one or more members selected from the groupconsisting of kaolinite, halloysite, pyrophyllite and cericite, and thepH of the adhesive being not more than 6, (d) the process comprisingapplying the adhesive, at a temperature of 60° to 95° C., onto the flutetips of corrugated medium, and utilizing the corrugated medium coatedwith the adhesive to produce a corrugated board.
 2. A process forproduction of corrugated board according to claim 1, wherein a viscosity(A) of the adhesive at 60° to 95° C. measured by a Hercules viscometerunder the shear rate of 3×10⁴ sec⁻¹ and a viscosity (B) of the adhesiveat the same temperature measured by a Brookfield viscometer have arelation of the following formula (iv) ##EQU8## and the viscosity of theadhesive measured by a Brookfield viscometer being in the range of 300to 3000 centipoises at 85° C.
 3. A process for production of corrugatedboard according to claim 1, wherein the viscosity measured by aBrookfield viscometer of a solution excluding solid other than polyvinylalcohol and water-soluble boron compound from the adhesive is 100 to1500 centipoises at 60° to 95° C.
 4. A process for production ofcorrugated board according to claim 1, wherein the clay has an averageparticle size of not more than 2 μm.
 5. A process for production ofcorrugated board according to claim 1, wherein the viscosity of theadhesive measured by a Brookfield viscometer is from 300 to 4000centipoises at 60° to 95° C.
 6. A process for production of corrugatedboard according to claim 1, wherein the polyvinyl alcohol comprises 10to 90 parts by weight of polyvinyl alcohol having a degree ofpolymerization of 200 to 800 and 90 to 10 parts by weight of polyvinylalcohol having a degree of polymerization of 1100 to
 3500. 7. A processfor production of corrugated board according to claim 1, wherein thepolyvinyl alcohol comprises 10 to 90 parts by weight of polyvinylalcohol (a) having a degree of hydrolysis of 72 to 100 mol % and 90 to10 parts by weight of polyvinyl alcohol having a degree of hydrolysislower by 2 to 30 mol % than that of (a).
 8. A process for production ofcorrugated board according to claim 1, wherein the adhesive furthercomprises at least one additive selected from the group consisting of aphosphorous compound, sodium polyacrylate, sodium alginate and guar gum,in an amount of 0.01 to 2 parts by weight to 100 parts by weight ofclay.
 9. A process for production of corrugated board according to claim1, wherein peroxide is added to the adhesive in an amount of 0.01 to 10parts by weight to 100 parts by weight of polyvinyl alcohol.
 10. Aprocess for production of corrugated board according to claim 1, whereinthe adhesive further comprises at least one additive selected from thegroup consisting of saccharose, colloidal silica, calcium carbonate,carbon black, white carbon, and silica anhydride, in an amount of 0.1 to20 parts by weight to 100 parts by weight of clay.